Ive been with this thread since the beginning. It's been an interesting ride with several turns but I dont think you should hang it up just yet.
Have you considered lining the walls with acrylic paneling and solvent cementing them together. The concrete walls would provide the strength (exoskeleton) to hold it together. You might be able to use .25 acrylic. I have done this before and it can work providing the shell does not flex. Since you have used masonry for the shell this idea might be worth a try.